Consumer electronics products are extremely price sensitive. Anything that can be done to reduce the cost of the product is very important. One way to reduce cost is to reduce the die size of the chip. Another way to reduce cost is to reduce the pin count of the chip and/or the number of components needed to support the chip.
In conventional DVD or CD-Recordable chip sets one timing loop (known as the read channel path) is used to recover user data from the disc. A second timing loop (known as the wobble data path) is used to recover pre-formatted information. These timing loops include analog loop filters and voltage controlled oscillators (VCOS). Duplicating similar timing loops uses additional die area, pin count, and passive components when compared to a single loop system.
One hallmark of a recordable DVD or CD disc is the preformatted position information encoded independently of user data. In contrast, a read only DVD and CD has coarse position information that is encoded in the headers of the user data blocks. Pits and lands representing user data are used to calculate the fine track error. The preformatted information is used to provide positioning information in the absence of user data. There is no user data on a new (or blank) disc.
Consequently, a recordable system needs a way to recover this preformatted information. A timing loop is implemented separately from the timing loop used to recover the user data. The particular details of the extra timing loop depends on the particular format of the recordable DVD/CD. Recordable media normally wobble the track slightly at a constant frequency to generate a clock signal used to synchronize the writing of data to the disc. The wobble ensures that the data is written to a known location. In addition to the wobble clock, coarse position information is written on the disc. The way the coarse information is written depends on the particular format of the recordable disc.
The CD-R and CD-RW formats use frequency modulation to encode a bit stream containing synchronization information and data block number. The DVD-R and DVD-RW formats use pre-formatted pits on the land adjacent to the track. Such land pre-pits (LPP) occur at particular times relative to the wobble. Detection involves opening a window in relation to the phase of the wobble. Finally, the DVD+R and DVD+RW formats encode the position data using a single cycle binary phase shift keying (BPSK) modulation method on the wobble so that a small set of the wobble cycles are opposite in phase to the rest.
Because the coarse position information is recorded in or synchronously to the wobble, the term “wobble data” is used to distinguish the coarse position information from user data. A common factor for each of the recording formats is that the recording system needs an accurate timing loop to (i) demodulate the encoded coarse position data and (ii) properly align the write data with the wobble.
Conventional systems use a timing loop dedicated to the wobble channel to accomplish such timing. Since the read channel also uses a timing loop, conventional systems implement two timing loops to simultaneously, or nearly simultaneously (i.e., one to read user data and one to demodulate the wobble data). Since each timing loop includes a phase detector, a loop filter, and a VCO, much of the circuitry in the device is redundant. The primary disadvantage of conventional solutions is that such solutions duplicate circuitry, which increases the cost of the device.
It would be desirable to implement a CD or DVD recording system that combines components from a read channel and a wobble channel to reduce the total number of components.